The basalt in Penghu Islands are mostly formed by the cooling lava from the ground fissures. It is known that the basalt were formed from about 17.4 million years ago to 8.2 million years ago. Two to four layers of lava can be seen on the surface; their geological time is middle to late period of the Miocene. The stratum is the clearest in Yuwengdao, so it is called the “Yuwengdao Layer”. Because it is spread all over Penghu Islands, it is called “Yuwengdao Layer Group” or “Penghu Layer”.
Penghu Geological Environment History
Penghu Islands Stratum Form
| Geological Time (years from now, million years) |
Stratum | lithologic character | ||
|---|---|---|---|---|
| CENOZOIC | QUATERNARY | Late Holocene (0.01) |
Modern beach deposit | Silica Sand, quartz wacke, cobble, coral, modern foraminifera, sell fragments |
| Early Holocene (0.01) |
Huxi Layer | Grayish yellow to grayish brown silty mudstone, below is fossil sand (modern shells) | ||
| Pleistocene (1.6) |
Xiaomenyu Layer | Coquina (Upper layer, part of it contains mammal fossil) and iron silica sand (lower layer) | ||
| TERTIARY | Pliocene(5.3) Late Miocene(8.2) To Mid Miocene(17.4) |
Penghu Layer | 1-3 layers of basal flow, 4 layers top. In between, there is sandstone, interbedding of mudstone and sandstone, tuff sandstone, or volcanic tuff-breccias | |
| Early Miocene To Oligocene(33.6)Eocene(57.8) Eocene(66.4) |
Baisha Islands Layer (subsurface formation) |
Mainly sandstone, and part of limestone | ||
| MESOZOIC | Creta-ceous To Jura-ssic |
(tentative)(208) | Baisha Islands Layer(subsurface formation) | Andesite-based volcanic lava; parts contain dykes and mudstone xenolith (Only in Huayu, the qualitive change of volcanic rock is about 60-65 million years) |
| Huayu(Huayu volcanic complex) | ||||
－paraunconformity -------unconformity。